This invention relates to acyclic machines employing liquid metal electrical current collectors, and more particularly to apparatus and method for providing a limited area liquid metal contact between the rotor and stator of such machines.
In acyclic machines utilizing high current density field coils, such as supercooled aluminum or copper coils cooled to the temperature of liquid nitrogen or superconducting field coils, high current-carrying capacity is required for making electrical connections between the rotor and stator. Liquid metal current collectors have been utilized for such machines due to their high current-carrying capacity.
Use of liquid metal in the current collectors results in viscous drag on the rotating rotor due to the contact with the liquid metal. By reducing the surface area of the liquid metal to rotor contact, the amount of viscous drag on the rotating rotor may be limited.
In the prior art is a technique disclosed in U.S. Pat. application Ser. No. 838,714, filed Oct. 3, 1977 by Hatch, now U.S. Pat. No. 4,146,807 and assigned to the instant assignee. In that application is disclosed a technique of limiting the rotor to liquid metal contact area by defining a narrow axial dimension and a limited circumferential dimension liquid metal contact by using a raceway-type confinement arrangement for the liquid metal current collector. The abutting raceways accommodate rotor eccentricity by shifting relative to the rotor whenever a raceway surface contacts the rotor. The frictional engagement then causes the abutting raceways to remain in the eccentric position until the rotor again contacts a raceway. Thereby, a slight variation in spacing between the raceway surface and the rotor is allowed.
Also known to me is a confinement structure as shown in FIG. 8a on page 272 of IEEE Transactions on Magnetics, Vol. MAG-13, No. 1, January, 1977 in an article titled "Superconducting Machinery for Naval Ship Propulsion" pp. 269-274, which utilizes an open-topped container, which rides on a layer of liquid metal between the stator and the containment device, and provides a liquid metal contact within the conductive container. This produces four liquid metal to solid metal contact surfaces and requires a substantial quantity of liquid metal to maintain each of the contact areas in complete contact.
In motor applications, such as for ship propulsion, capability of applying full torque (and overload torques) at all and any motor speeds (e.g., up to 1500 RPM) in both directions of rotation, including zero speed and emergency reversals, is required. To carry the necessary current at all modes of operation a continuous supply of liquid metal is required at each collector site. This requirement in addition to the requirement of limiting liquid metal to rotor surface area to limit viscous losses is achieved by the invention disclosed herein by containing a continuous supply of liquid metal under pressure within an enclosure at each collector site.
An additional problem experienced in utilizing liquid metal current collectors in acyclic machines is that the intense magnetic field from the field coils creates a voltage across the surfaces of the rotor collector rings in the axial direction, tending to eject the liquid metal from the collector site. If the liquid metal is allowed to be moved from the current collector site, the current collectors would run either partially dry raising the current density to a point where arc-over between the rotor and stator collector areas would occur, or entirely dry causing a break in the electrical connection. In either case, the acyclic machine would be rendered inoperative. Therefore, in one embodiment of the instant invention, permanent magnet inserts are provided in the enclosure wall to locally reduce and align the magnetic field at the collector sites, thereby allowing higher current densities to be carried by a given current collector.
Accordingly, one object of the invention is to provide a liquid metal current collector configuration which allows continuous electrical contact between a predetermined area of a stator collector in an acyclic machine and the rotor collector ring thereof.
Another object of the invention is to provide a liquid metal current collector which exhibits reduced viscous drag losses at high machine speed while providing continuous contact at all modes of machine operation.
Another object of the invention is to provide permanent magnets in the enclosure wall to reduce the magnetic effects upon the liquid metal in the current collectors.